A conventional electrostatic chuck includes an insulation substrate, an electrostatic electrode, and a base plate. The electrostatic chuck has a gas passage penetrating the insulation substrate, the electrostatic electrode, and the base plate in a thickness direction of the insulation substrate, the electrostatic electrode, and the base plate, respectively (see, for example, FIG. 2 of Japanese Laid-Open Patent Publication No. 2010-153490).
In a state where, for example, a wafer is placed on the insulation substrate of the conventional electrostatic chuck, an abnormal discharge may occur in the gas passage when generating plasma on a surface of the wafer by supplying high frequency electric power to the base plate formed of metal.
This abnormal discharge is due to the exposure of the inner wall of the gas passage of the base plate.